Signal translating apparatus



p 1 0- F. MASSA 2,214,591

PE)? CENT EFFICIENCY SIGNAL TRANSLA'I'ING APPARATUS Filed Nov. so, 1936 's Sheets-Sheet 1 a A I Z; A 1 r I W 60 fig 40 I TE 1 i 7 Frwnhi Macaw attorney I FREQUENCY l/V CYCLES.

su p 10, 1940. v F. MASSA 2,214,591

SIGNAL TRANSLATING APPARATUS Filed Nov. 30, 1936 5 Sheets-Sheet 3 Zhwentor Fran 7i Massa Wdttomeg Patented Sept. 10, 1940 PATENT OFFICE 2,214,591 SIGNAL TRANSLATING APPARATUS Frank Massa, West Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 30, 1936, Serial No. 113,475

1 Claim.

This invention relates to signal translating apparatus, and more particularly to horn loudspeakers employing electrodynamic drivers.

It is well known that for the reproduction of 5 low frequencies, it is best to employ a loudspeaker having a large diaphragm in order to radiate acoustic energy with minimum distortion. On the other hand, for the reproduction of high frequencies, it has been found desirable to employ in a loudspeaker having a small diaphragm in order to prevent the diaphragm from breaking up, since it is generally desirable to have the diaphragm vibrate as a piston so that subharmonics will not be generated. It has also been well recognized 16 that a horn improves the radiation efficiency of a loudspeaker and for this reason many loudspeakers have been provided with horns.

Various proposals have heretofore been made to provide horn loudspeakers in which advantage '1 is taken of the foregoing principles. According to some, two entirely separate speakers are provided, one especially for reproduction of low frequencies and another especially to reproduce high frequencies, and each feeding into aseparate ll horn. This type of loudspeaker has the disadvantage of requiring separate field structures for each diaphragm, if of the .electrodynamic type, and the further disadvantage that two separate horns must be employed. According to other a proposals, loudspeakers have been formed with either a single diaphragm and a single horn or a multi-section diaphragm and a single horn in order to cover the entire frequency range. In such cases, the horn must of necessity be designed to I be a compromise between a good low frequency horn and a good high frequency horn. Accord-.

ing to still other proposals, a single diaphragm is employed to feed what is known as a compound horn, or two separate horns, one associated with a the front surface of the diaphrgam and especially designed for high frequency reproduction, and the other associated with the rear surface of the diaphragm and especially designed for low frequency reproduction. Here, again, a compromise must be struck to obtain good response over the entire audio frequency range because of the limitations imposed by the use of a single diaphragm. In general, the diaphragm is too large to give optimum high frequency performance and too small 50 to give good low frequency performance. Moreover, since, as in conventional loudspeakers of this type, a single voice coil is employed, both high and low frequency currents must be fed into the same coil, and acoustical means must be pro- 55 vided for causing a proper division of frequencies between the two horns. Another disadvantage of the single diaphragm type of compound horn loudspeaker is that while the diaphragm is radiating high frequency vibrations, it is simultaneously being supplied with low frequency currents 6 which cause it to move through large, low frequency excursions as well. This dual movement usually produces distortion in the high frequency output. I

The primary object of my present invention is ID to provide an improved horn loudspeaker which will be free from the aforementioned disadvantages characteristic of prior art loudspeakers.

More specifically, it is an object of my present invention to provide an improved horn loudspeaker which will provide improved overall performance over that which has heretofore been obtained from' a single loudspeaker.

Another object of this invention is to provide an improved horn loudspeaker which has a high- 59 .er power handling capacity than is obtainable from corresponding prior art loudspeakers of this type.

1 Still another object of my present invention is to provide an improved horn loudspeaker which gs is characterized by a minimum of distortion, and particularly by a reduction of phase distortion in the horn throat.

A further object of my present invention is to provide an improved horn loudspeaker in which 0 the division of frequencies between the high frequencies and the low frequencies can be more accurately controlled than heretofore.

Astill further object of my present invention is to provide an improved horn type electrody- 35 namic loudspeaker in which a single field structure is employed to most efficiently supply flux for a plurality of armatures or driving coils.

It is also an object of my present invention to provide an improved horn type loudspeaker as o aforesaid which is relatively compact in construction, economical to operate, and highly efficient in use over the entire audio frequency range.

In accordance with my present invention, I employ two separate vibrating systems, one for the 5 high frequency region and one for the low frequency region. Specifically, I employ two separate voice. coils and two separate diaphragms or cones, a large cone with a large voice coil for driving a preferably folded, low frequency horn g and a small cone with a small voice coil for driving a straight axis, high frequency horn which may be nested within the low frequency horn, if desired. Thus, I can, without compromise between good high frequency reproduction and good 5 a range of approximately 30 to 10,000 cycles and low frequency reproduction, independently choose a suitable cone and voice coil to provide most efficient low frequency performance and another cone and voice coil suitable to most efficient high frequency performance. I also employ a common field structure to supply flux to both voice coils and arrange it in such a manner that the flux in the air gap for the high frequency voice coil is more dense than that in the air gap for the low frequency voice coil, which is a condition that I have indicated in my copending application Serial No. 113,120 as being very desirable in order to obtain maximum efficiency at the higher frequencies. As a further feature of my present invention, I can feed only high frequency voltages to the high frequency speaker, and low frequency voltages to the low frequency speaker, dividing the power either electrically, acoustically, or in both ways. Such a division of power makes it possible to accurately control the overlap region.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof will best be understood from the following description of several embodiments thereof when read in connection with the accompanying drawings wherein Figure 1 is a graph showing several eiiiciency curves and horn loudspeakers of the type under consideration,

Figure 2 is a central sectional view of one form 5 of horn loudspeaker built according to my present invention,

Figure 3 is a similar View of another form of my invention,

Figure 4 is a similar view of still another and preferred form of my invention, and

Figure 5 is a circuit diagram showing one method of electrically dividing the power supplied to the voice coils.

In my aforementioned copending application,

I have fully discussed the fundamental factors which limit the performance of electrodynamically driven horn loudspeakers and have therein shown two families of curves showing maximum possible efficiencies that can be realized at various frequencies under ideal conditions. The conclusions reached as a result of the analysis set forth in my said copending application are that the efiiciency of an electrodynamic horn loudspeaker is dependent upon the product of the resistivity times the density of the material of the voice coil, that efficiencies at the lower frequencies are inherently high, and that at the higher frequencies, the efficiency is critically dependent upon the ratio of the voice coil mass to the mass of the remainder of the vibrating system.

Referring, now, more specifically to the accompanying drawings, and particularly to Figure 1, the curve A shows the maximum efficiency obtainable from a horn loudspeaker over represents one of the curves of the family of curves referred to above in connection with my previously mentioned copending application. From the curve A, which represents ideal conditions and was plotted with a fixed flux density at the air gap and on the assumption that the horn impedance is matched to the cone or diaphragm impedance at each point in the range covered thereby, it will be noted that the chiciency at the lower frequencies is very high and that it drops off gradually as the higher frequencies are approached. In actual practice, it is, of course, entirely impractical for the foregoing condition to prevail in a single horn, and, therefore, in order to approach this condition, a number of horns must be employed. The curve B represents the efliciency characteristic of the loudspeaker where the throat area of the horn is such that the horn impedance matches the diaphragm impedance at about 100 cycles. Here, it will be seen that the eiiiciency at the lower frequencies remains substantially the same up to about 200 cycles, after Which the efficiency drops much more rapidly than in the case assumed for the curve A. Coming, now, to the curve C, this shows the eificiency of a horn loudspeaker having a relatively small throat area and the horn impedance of which matches the diaphragm impedance at about 7,000 cycles. In this case, it will be seen that the eiliciency is quite low until about'700 cycles after which it also drops off but at a much slower rate than in the case of the curve B and more nearly like the curve A. It is obvious, therefore, that a combination of two separate diaphragms and two separate horns, one combination of which follows the curve B and the other of which follows the curve C, will provide a compound horn loudspeaker which will follow the curve B to the point P (at which the curves B and C intersect) for most efficient reproduction of the lower frequencies, and thereafter will follow the curve C for most efiicient reproduction of the higher frequencies, thereby approaching more nearly the ideal curve A. It is upon this principle that the several modifications of my present invention now to be described in detail are based.

Referring, first, to Figure 2 of the drawings, I have shown a compound horn loudspeaker having a relatively long horn i formed of a plurality of concentric, reversely directed, tub-like sections la, lb and lo preferably arranged to expand substantially exponentially and adapted to particularly accentuate the lower frequencies within the audio frequency range, and a relatively short horn 3 nested concentrically within the horn section la and adapted to particularly accentuate the higher frequencies within the audio frequency range, the horn 3 being a straight-axis horn and being also preferably formed to expand substantially exponentially. The horn 3 is suitably held in place within the horn section la by means of a plurality of partitions 5 and 'l which are secured to both the horn 3 and to a lining member 9 which, in turn, is secured to the horn section la. A plurality of holes II and iii are formed in the partitions 5 and 1, respectively, circumferentially therearound for a purpose presently to be described.

Secured to the inner end of the lining member 9 is an apertured frame or dish-pan l5 which flexibly supports a relatively large irustoconical diaphragm or other suitable vibratile member I! provided with a cylindrical coll form IS on which a conductive voice coil or armature 21 is wound in conventional manner. To the frame 15 is suitably secured an annular plate 23 of magnetic material which is united with a U-shaped yoke 25, as by welding or soldering, the yoke 25 having a central pole piece 21 riveted or otherwise suitably secured thereto, and a coil 29 serving to magnetize the pole piece 21 when energized. A ring 3|, which is U-shaped in cross-section and is made of non-magnetic material, such as brass or copper, for example, serves to connect the plate 23 with a second annular plate 33 of magneticmaterial, the plates 23 and 33 being in the same plane and being radially spaced from each other to provide an annular air gap for the voice coil 2!. The annular plate 33 is also spaced from the reduced end of the center pole piece 21 to provide a second airgap 31. which is concentric with the air gap 35 and receives a conductive voice coil or armature 39 which is suitably wound on a voice coil form 4| of a relatively small diaphragm 43 flexibly held against the rear of the horn 3 .by a ring 45 which is interposed between the plate 33 and the born 3.

It will be noted from the foregoing that the air gaps 35 and 31 are in series and that the flux coming from the central pole piece 21 passes through "both of them, the flux density within the air gap 35 being weaker, however, than that in the air gap 31 by reason of the fact that the mean cross sectional area of the air gap 35 normal to the direction -of flux is more than the corresponding area of the air gap 31. The voice coil 2| is large and preferably made of copper, while the voice coil 39 is small and preferably made of aluminum (although either could also be made of the other material under certain conditions) in accordance with the teachings of my aforementioned copending application wherein I have pointed out that the efiiciency of any system such as herein described is dependent, among other things, upon the density-resistivity product of the material of the voice coil. It is for this reason that 'I choose materials of different density-resistivity products for the voice coils 2| and 39.

The relatively large voice coil 2| is designed to be particularly responsive to low frequencies and serves to drive the large cone [1, which, in turn, feeds into the large horn l which has a relatively large throat area and the impedance of which may match the impedance of the cone l1 at about 100-150 cycles. On the other' hand, the relatively small voice coil 39 is designed to be particularly responsive to high frequencies and serves to drive the small cone 43 which feeds into the small, high frequency born 3, the latter having a small throat and having an impedance which matches the impedance of the cone 43 preferably in the range between 2000-7000 cycles. In this way,'the elliciency of the entire system may be made to follow closely along such curves as the curves B and C of Fig. '1.

In order to insure a proper electrical division of energy between the voice coils 2| and 39 when they are fed from a common source of audio frequency signals, the current therefrom may be fed through a low pass filter 41 to the coil 2| and through a high pass filter 49 to the coil 39, as clearly shown in Fig. 5. Further control of the rangeto' be reproduced through the horn i may be. controlled by the aforementioned openings H and I3 in the partitions 5 and 1 which not only serve to establish communication between the cone l1 and the horn l'but which may be suitably proportioned to constitute a low pass acoustic filter which will give the low frequency horn I a cut off at some predetermined frequency which has been found desirable. At the same time, the cut-off frequency of the high frequency horn 3 due to its flare may be arranged to fall near the frequency at which the division of electrical energy as shown in Fig. 5 is made. Thus,

it is possible to fairly critically control the overlap region between the horns I and 3, and particularly so since the rings 3| and 45 prevent air displacements set up by either the diaphragm l1 or'the diaphragm 43, upon vibration thereof, from reaching the other one. At the same time, by closing off the chamber or cavity behind large cone H, as by means of a wall 5i, this chamber may be made to resonate with the cone l1 at a frequency below the lowest frequency to which it is responsive (or to a frequency not appreciably thereabove), whereby the low frequency system described will-have a substantially flat response characteristic throughout its entire working range.

, In Fig. 3, I have shown a, system which is very similar to that shown in Fig. 2. In the modification of Fig. 3, the horn I is provided with an additional, innermost section Id to which the small horn 3 is directly secured at the front end with the small endof the horn 3 held against its throat 3a supported within the section id by a plurality of bracket members 53. The small cone 43 is flexibly supported against the throat 3a, as in the modification of Fig. 2, by the ring 45, while the large cone I1 is flexibly supported between a ring member and the frame 15.

The magnetic structure of the modification under consideration comprises a drumlike member 51 of magnetic material to which are secured the plates 59 and 61, also of magnetic material and corresponding respectively to the plates 23 and 33 of Fig. 2, each of the plates-59 and 6| being provided with a plurality of circumferentially spaced apertures 63, whereby communication is established between the rear surface of the cone l1 and the chamber between the small horn 3 and the horn section Id. This chamber may be made- .to resonate with the cone il in a manner previously described in connection with Fig. 2 to provide a substantially flat response characteristic for the low frequency system.

The central pole piece 21 is held in place by a pair'of'ringlike members 61 and 59 of nonmagnetic material, each firmly engaging the centrol pole piece 21 and being suitably secured to their respectively associatedplates 59 and 6!. Like" the rings 3! and 45 of Fig 2, the rings 61 and 69 of Fig. 3 serve to prevent air displacements set up upon vibration of either of the cones l1 and 43 from reaching the other.

It will be noted that the plate 59 is considerably thicker than the plate GI and that the central pole piece 21 isof much smaller diameter at the end adjacent the horn 3 than at its other end. Since the air gaps 35and 31 are again in series; it will be obvious that the flux flowing through the air gap 31 is necessarily much more dense than that flowing through the. air gap 35. This is a condition necessary to greater efficiency at the higher frequencies and can be very easily realized in a construction such as that described by reason of the fact that the central pole piece 21 is common to both air gaps.

In Fig. 4 of the drawings, I have shown another modification of my invention wherein a plurality of high frequency horns 3 are employed in order to obtain greater spread of the high frequencies. The horns 3 preferably expand exponentially and are substantially straight axis horns, having only very slight bends therein in order to effect a proper distribution of the vibrations emanating therefrom over a large area. The smaller ends of the horns 3 are supported in a cup-shaped supporting member 1i within which the throats 3a of the horns 3 may be fitted, a frame member I3 serving to flexibly hold the small cone 43 against the throat 3a. The arrangement of the field structure for supplying flux to the air gaps 35 and 31 may be similar to that already described in connection with Fig. 3 and therefore is not believed to require further explanation.

From the foregoing description, it will be apparent that I have provided an improved form of compound horn loudspeaker wherein it is not necessary to compromise between either good low frequency or good high frequency performance, the large diaphragm I! with its copper voice coil feeding the low frequency, large-throat, folded horn I most efficien'tly, and the small diaphragm 43 with its aluminum voice coil feeding the high frequency, small-throat, straight-axis horn most efiiciently. By employing the large cone II in the manner described, I am able to obtain not only greaterlow frequency efiiciency, but also increase power handling capacityand lower distortion,

while the use of the small cone 43 as the high frequency driver enables me to obtain not only greater eificiencies at the high frequencies, but an extended high frequency range and a reduction of phase distortion in the throat of the high frequency horn 3 (or horns 3, as the case may be) as well. Due to the use of separate voice coils for the low and high frequency units, it is possible to divide the audio frequency supply electrically between the two speaker components, as fully described heretofore. This permits a definite control of the frequency division between the two systems. The useof an acoustic filter further enables better control of the division of the reproduced frequencies, and an acoustic filter of the type described in connection with Fig. 2 may, obviously, also be employed in connection with the modifications of Figs. 3 and 4. I also wish to point out that the use oi. filters such as shown in Figure 5 in connection with the two separate speakers, as described will prevent low frequency currents being impressed on the high frequency voice coil 39 and the possible distortion which may occur in a single cone system in which the high and low frequencies are combined is entirely eliminated.

Although I have shown and described several modifications of my invention, I am fully aware that many further modifications thereof are possible and that many changes may be made therein without departing from the spirit of the invention. I, therefore, desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claim.

I claim as my invention:

In signal translating apparatus, the combination of means providing a chamber, a relatively large vibratile member closing one end of said chamber, a driving coil'particularly responsive to low frequencies associated with said large member, a relatively small vibratile member, a second driving coil particularly responsive to' high frequencies associated with said small member, and means common to both of said coils for providing a magnetic field to coact with said coils, said means being so constructed and arranged as to provide a serial flux circuit for both said coils and a more dense field for said second named coil than for said first named coil, and said chamber resonating with said large vibratile member in a region such that a substantially fiat response characteristic is provided for said large vibratile member.

FRANK MASSA. 

